1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing a semiconductor device, and more particularly, to a semiconductor device whose structure is suitable for reducing a threshold value for the operation of a transistor thereof, as well as a method of manufacturing the semiconductor device.
2. Description of the Background Art
Static random-access memory (SRAM) has been known as one example of a semiconductor device including transistors. SRAM is volatile semiconductor memory and comprises a plurality of bit lines and a word lines, arranged in a matrix pattern, and a plurality of memory cells formed at respective points of intersection of the bit and word lines.
Each memory cell comprises two access transistors and two driver transistors. Each of the two driver transistors is connected to a load element for receiving a power voltage. The load elements and the driver transistors constitute a flip-flop circuit which has two memory nodes and brings the two memory nodes into one of two stable states.
Each of the two access transistors is disposed between the memory node and one of the pair of bit lines. One of the bit line pair will hereinafter be represented as xe2x80x9cBIT,xe2x80x9d and the remaining one of the bit line pair will hereinafter be represented as xe2x80x9c/BIT.xe2x80x9d In SRAM, data are supplied to each memory cell from the bit line pair by way of the access transistors, so that the data are stored in the flip-flop circuit (memory node). On the other hand, the data stored in each memory cell can be read when the potential of the memory node is led to the bit line pair by way of the access transistors.
An effective way to stably activate SRAM is to set a threshold value Vath for the operation of the access transistors to a small value and a threshold value Vdth for the operation of the driver transistors to a large value. For these reasons, as described in Japanese Patent Application Laid-Open No. Hei4-61377, the threshold value Vdth for the operation of the driver transistors is commonly set to become higher than the threshold value Vath for the operation of the access transistors in SRAM.
In conventional SRAM, the threshold value for the operation of a transistor is controlled by the amount of impurities doped into a semiconductor substrate. Specifically, a p-type impurity concentration of an area immediately below the gate electrode of the driver transistor is controlled so as to become greater than the p-type impurity concentration of an area immediately below the gate electrode of the access transistor, thereby rendering the threshold value Vdth greater than the threshold value Vath.
In order to make the concentration of impurities implanted into the semiconductor substrate different from area to area, a photoresist must be patterned such that areas whose impurity concentration is to be reduced are covered with the photoresist, and impurities must be implanted into the semiconductor substrate while the photoresist is taken as a mask. In this respect, the conventional method encounters the problem of increasing costs incurred by manufacture of SRAM.
As the access and driver transistors become miniaturized in association with integration of SRAM, the intervals between the transistors become smaller, thereby diminishing a margin for photolithography. Accordingly, openings to be formed in the photoresist are likely to become formed in the areas where access transistors are to be fabricated. In this case, the threshold value Vath for the operation of the access transistors is increased, so that the operation of SRAM becomes unstable.
The present invention has been conceived to solve such a problem in the conventional method, and the object of the present invention is to provide a semiconductor device which is suitable for inexpensively fabricating transistors thereof whose threshold value for operation is small, as well as a method of manufacturing a semiconductor device.
The above objects of the present invention are achieved by a semiconductor device having a field-oxide film for insulating a plurality of active regions from one another. The device includes first and second active regions which are to act as source/drain regions of a transistor. The device also includes a third active region which is to be interposed between the first and second active regions and act as a channel region of the transistor. The field-oxide film has a protuberance in a boundary area with the first through third active regions, so as to protrude toward the third active region. A trench is formed in a boundary region between the protuberance and the third active region.
The above objects of the present invention are also achieved by a method of manufacturing a semiconductor device having a field-oxide film which insulates a plurality of active regions from one another. In the inventive method, on a silicon substrate is formed a pad film from a silicon oxide film. An oxidation-resistance mask of predetermined pattern is formed on the pad film. A field-oxide film is formed in areas exposed from the oxidation-resistance mask by means of subjecting the silicon substrate to thermal oxidation. Transistors are fabricated after removal of the oxidation-resistance mask, by utilization of active regions covered by the oxidation-resistance mask. The oxidation-resistance mask is formed in an area which covers a source/drain region and a channel region of a specific transistor, so as to have a concave portion depressing toward the channel region when viewed from above.